1. Field of the Invention
The present invention relates to an apparatus for correcting asymmetry existing in signals to be reproduced from an optical disc, and more particularly, to an apparatus for correcting asymmetry of reproduced signals through a signal processing in digital form.
2. Description of the Related Art
Digital data recorded on an optical disc exists physically in the form of pits. Since it is difficult to form the pits ideally, each length of the pits may be axially increased or decreased. This phenomenon is called an asymmetry. The asymmetry occurs due to a variety of causes such as a manufacturing process and a performance of a pickup, which is an unavoidable factor in a disc handling process. The asymmetry turns up as an asymmetry component in a reproduced signal, thereby causing accuracy of an optical disc system to be lowered.
FIG. 1 shows a reproduction signal which has been sampled from an optical disc using an optical disc system. In FIG. 1, it can be seen that a level of the sampled reproduction signal has been shifted on the vertical axis by 4V corresponding to a DC offset of +2V entirely. Accordingly, it can be seen that an asymmetry component exists in the reproduction signal.
FIG. 2 shows a conventional analog asymmetry correction apparatus for correcting asymmetry in an optical disc system. In the FIG. 2 asymmetry correction apparatus, a comparator 10 compares a signal read from an optical disc (a reproduction signal) with a feedback reference signal and outputs binary non-return-to-zero-inverted (NRZI) data. An integrator and low-pass-filter 11 calculates a digital sum value (DSV) from the binary data, removes a local peak value from the calculated DSV and feedbacks the result to the comparator 10 as a reference value. The conventional correction apparatus maintains the reference value of the comparator 10 so that the DSV of the reproduction signal is minimized and the reference level for detecting the reproduction signal is maintained to be a central value of the input reproduction signal, as shown in FIG. 1. Through the above process, the asymmetry component with respect to the reproduction signal is corrected.
The optical disc system having the analog asymmetry correction apparatus needs a digital signal detector rather than an analog signal detector, according to an increase in a recording density of the optical disc. Further, the optical disc system needs an asymmetry correction apparatus for adaptively correcting asymmetry with respect to the recorded digital signal and signal-processing the same in digital form.
However, the conventional analog asymmetry correction apparatus has limited processing speed and capacity when correcting digital signal asymmetry according to an increase in a recording density on an optical disc.
Also, in the case of a DVD-RAM where a discontinuous interval exists in a reproduction signal, the conventional asymmetry apparatus does not change a time constant of an integrator at the right time in correspondence to a different amount of asymmetry at every interval. Further, when a short interval such as a header interval exists, the conventional asymmetry correction apparatus has the difficulty in changing a time constant of the integrator in a timely fashion.